Paleoenvironments and relative sea-level changes caused by regional tectonics during the last 4500 years in Kumihama Bay, northern Kyoto Prefecture, central Japan

Abstract

Ostracode assemblages were examined, and the grain size and chemistry of sediments were analyzed with the aim of clarifying paleoenvironmental and relative sea-level (RSL) changes relating to regional tectonics in Kumihama Bay, central Japan. Overall, 21 genera and 38 species of ostracode were identified from two cores taken in the northwestern and southeastern parts of the bay. The analysis of sedimentary facies, ostracode assemblages and geochemical records of sediments revealed the following environmental changes. A closed bay, with an area smaller than that of today, existed between 4500 and 4300 cal BP. A gradual increase in seawater influence in the bay was caused by an RSL rise between 4300 and 2000 cal BP with highest RSL occurring at 2000 cal BP. Between 4000 and 2500 cal BP, the RSL in Kumihama Bay has shifted from −3.8 m to −2.0 m at a rate of 1.2 mm/yr. This rate is inconsistent with the predicted RSL trend in surrounding regions, suggesting the occurrence of a subsidence in Kumihama Bay during the Late Holocene. The trend in tectonic movement and their rate are common in the San'in shear zone, implying that this subsidence might be related to the subducting of the Philippine Sea Plate.